1. Field of the Invention
The present invention relates to an optical spectrum analyzer, and more particularly, to an optical spectrum analyzer that employs a technology for improving wavelength accuracy of spectrum characteristics of light when the spectrum characteristics are determined by using a tunable wavelength filter.
2. Description of the Related Art
Optical spectrum analyzers for determining spectrum characteristics of light determine a relation between wavelength and intensity as spectrum characteristics of to-be-measured light, that is, spectrum data by detecting intensity of light that is caused to exit from a tunable wavelength filter on which the to-be-measured light is incident while changing a wavelength selected by the tunable wavelength filter.
It is required as performances of the optical spectrum analyzer that spectrum data as spectrum characteristics to be determined has high wavelength accuracy and high resolution, a wavelength can be measured in a wide range, a wavelength can be swept at high speed, and the like.
These performances of the optical spectrum analyzer are mainly determined by the performance of a tunable wavelength filter used to the optical spectrum analyzer.
A Fairy-Perot filter, which is a kind of a so-called Entaslon, is known as the tunable wavelength filter used conventionally in the optical spectrum analyzer.
As shown in FIG. 16, the Fairy-Perot filter has a so-called cavity configuration in which a pair of optical elements 1 and 2 are arranged in parallel in confrontation with each other, and the light having a wavelength component, which is determined by a gap d between the pair of optical elements 1 and 2, of light Pa incident on the one optical element 1 from the outside is caused to selectively exit to the outside of the other optical element 2.
In the Fairy-Perot filter, a wavelength of the light Pb outgoing from the optical element 2 can be changed by changing the gap d between the pair of optical elements 1 and 2.
In the tunable wavelength filter composed of the Fairy-Perot filter configuration, it is known that a relation of 2nd=mλ (m is integer) is established between the gap d and the wavelength of outgoing light, wherein n denotes a refraction factor of the pair of optical elements 1 and 2.
When the tunable wavelength filter is actually composed of the Fairy-Perot filter having the pair of optical elements 1 and 2, a moving mechanism is necessary to fix one of the pair of optical elements 1 and 2 and to minutely move the other optical element 2 in parallel with the optical element 1 in order to change the gap d between the pair of optical elements 1 and 2.
As the moving mechanism, there is known a moving mechanism as shown in FIG. 17 which is arranged by applying an etching technology, that is, a so-called MEMS (Micro-Electro-Mechanical-systems) technology to a semiconductor substrate and the like (Patent Document 1: U.S. Pat. No. 6,373,632).
In the moving mechanism exemplified in FIG. 17, a disc plate 6 acting as one of a pair of optical elements is formed at the center of a flat frame-shaped substrate 5, and further the inside edge of the substrate 5 is coupled with the outside edge of the disc plate 6 by a plurality (four in FIG. 17) of flexible thin beams 7, 7 . . . that protrude toward each other.
In the moving mechanism, for example, a voltage is applied between the disc plate 6 and a fixed electrode (not shown) confronting the disc plate 6, and the disc plate 6 is moved forward or backward (in a direction orthogonal to the sheet in FIG. 17) by the electrostatic attracting force of the voltage, thereby a gap between the disc plate 6 and a fixed optical element (not shown) acting as the other of the pair of optical elements can be changed.
In an optical spectrum analyzer using the tunable wavelength filter composed of the Fairy-Perot filter, spectrum data of to-be-measured light can be obtained by determining intensity of light, which is selected by the tunable wavelength filter and caused to be incident on a light receiving unit, by the light receiving unit.
However, the optical spectrum analyzer using the Fairy-Perot filter as the tunable wavelength filter has a problem in that a wavelength cannot be increased in a wide range in principle.
More specifically, a wavelength λ of outgoing light is shown by the following equation from a relation between the gap d described above and a wavelength.λ=2nd/mSince outgoing light has a plurality of wavelengths to the same gap d depending on a value of m, the wavelength of the outgoing light cannot be uniquely determined.
FIG. 18 shows a relation between a wavelength the gap d when m=1 to 4.
In FIG. 18, when it is assumed that a desired wavelength is within a range of λ1 to λ2 and the gap is set within a range of d1 to d2 to realize the wavelengths λ1 to λ2 when m=1, three different components having wavelengths 2nd, nd and 2nd/3 are selected with respect to the same gap d when the gap is near to d2.
To prevent the above drawback, the lower limit of the wavelength must be increased from λ1 to λ1′=nd2, by which the wavelength changeable range of the optical spectrum analyzer is restricted because the wavelength changeable range is greatly reduced.
Further, in the optical spectrum analyzer using the Fairy-Perot filter as the tunable wavelength filter, a high degree of parallelism is required to the pair of optical elements to set wavelength selection characteristics within a narrow band.
However, as described above, in the structure in which the disc plate 6, which corresponds to one of the pair of optical elements, is supported through the plurality of thin beams 7, 7 . . . as in the moving mechanism formed by the MEMS exemplified in FIG. 17, the disc plate 6 is inclined even by a minute difference between the plurality of the beams 7, 7 . . . , thereby it is difficult to obtain narrow band characteristics.
To overcome the above problem, since it is necessary to increase the number of electrodes for moving the disc plate 6 to minutely control an attitude of the disc plate 6, thereby a structure is made complex and it is difficult to change a wavelength at high speed.
Further, a problem also arises in that the attitude of the disc plate 6 is liable to be changed in structure by a change of temperature and humidity and accuracy of an outgoing wavelength is deteriorated thereby.
Incidentally, in the optical spectrum analyzer as described above, spectrum data is determined by causing reference light whose wavelength is known to be incident on the tunable wavelength filter in place of the to-be-measured light and the wavelength axis of the spectrum data is calibrated based on the known wavelength of the reference light in order to maintain the accuracy of wavelength of the obtained spectrum data.
However, when the optical spectrum analyzer is used in a place in which an environment is intensely changed, an optical system is changed by the change of the environment, from which a problem may arise in that the accuracy of wavelength of the obtained spectrum data is greatly deteriorated.
Accordingly, the optical spectrum analyzer itself must be calibrated frequently, from which a problem arises in that it is difficult to continuously measure to-be-measured light that is to be continuously measured intrinsically.